A prior art waveguide orthomode transducer is provided with a main waveguide including a metallic thin plate disposed at its branch portion, the metallic thin plate having circular notches which are so formed as to be bilaterally symmetric with each other.
Since this metallic thin plate is so formed, a horizontally polarized electric wave H of a basic mode inputted to the waveguide orthomode transducer via an input terminal P1 is branched into two routes which are right-angled and symmetric with respect to the direction of the axis of the main waveguide, and the two parts are outputted from output terminals P3 and P4, respectively.
On the other hand, a vertically polarized electric wave V of a basic mode inputted to the waveguide orthomode transducer via the input terminal P1 is outputted from another output terminal P2 which is opposite to the input terminal P1 (refer to patent reference 1, for example).    [Patent reference 1] JP,11-330801,A (refer to pp. 4 to 6 and FIG. 1)
A problem with the prior art waveguide orthomode transducer constructed as mentioned above is that since a metallic thin plate is inserted into the branch portion of the main waveguide, the length of the main waveguide along the direction of the axis becomes long and it is therefore difficult to do miniaturization of the waveguide orthomode transducer with respect to the direction of the axis of the main waveguide and to reduce the length of the main waveguide along the direction of the axis.
Since there is a large change in the wavelength of electric waves in the waveguide with respect to frequency in a frequency band in the vicinity of the cut-off frequency of the vertically polarized electric wave of the basic mode and the horizontally polarized wave, and there is also a rapid change in the impedance discontinuity with respect to frequency at the branch portion of the main waveguide, it is difficult to suppress degradation in the reflection characteristics of both the vertically polarized wave and the horizontally polarized wave in the frequency band in the vicinity of the cut-off frequency.
The present invention is made in order to solve the above-mentioned problems, and it is therefore an object of the present invention to provide a high-performance waveguide orthomode transducer that can come down in size and can have a waveguide whose axis is downsized.